Present a popover on tapped cell

## Expected Behavior

## Actual Behavior

While presenting a popover over a cell that was tapped on, doing this should give the rect of the tapped cell -
```
let rect = self.gradebookView.rectForItem(at: indexPath)
let rectInView = self.gradebookView.convert(rect, to: self.view)
```
but it seems the rect being obtained after converting it to the parent view's coordinate is not right and the popover always ends up getting opened someplace else and sometimes even out of the view.

## Steps to Reproduce the Problem

1. Tap on a cell
2. Get the rect of the cell in didSelectItemAt:indexPath and convert that rect to the parent view's coordinates
3. Presenting the popover with that sourceRect shows the popover over a completely different cell.

## Specifications

- Xcode version: 9.1
- iOS version: 11.1
- Dependency manager + version: 0.8.4